The invention relates to a process for minimizing the formation of low molecular weight oligomers during the hydrolytic depolymerization of condensation polymers.
The continuous hydrolysis of high molecular weight condensation polymers such as polyethylene terephthalate is well known in the art. Terephthalic acid and ethylene glycol are the primary products of polyethylene terephthalate hydrolysis, a process for the reversal of its polymerization. A by-product produced during such hydrolysis is p-mono(2-hydroxyethyl) terephthalate (MHET) which has the formula: ##STR1## The presence of MHET and other low molecular weight oligomers adversely affects the depolymerization process in three ways:
(1) it interferes with the crystallization of one of the desired products, i.e., terephthalic acid; PA1 (2) it creates difficulties in the liquid-solid separation stage, i.e., isolation of the terephthalic acid from the liquid phase; and PA1 (3) it decreases the yield of the desired end products, i.e., terephthalic acid and ethylene glycol. PA1 (1) the amount of water and the condensation polymer present in the hydrolysis zone are such that the least soluble final depolymerization product would not exceed its solubility limits, assuming all of the polymer is converted to final depolymerization products; and PA1 (2) temperature conditions and aqueous concentration of final depolymerization products are controlled so that at equilibrium, the amount of low molecular weight oligomers is no greater than 7% of the theoretical, based on the amount of polymer added. PA1 where M=moles water/moles ethylene glycol; and PA1 T=weight terephthalic acid/weight water PA1 M=moles water/moles ethylene glycol PA4 (after hydrolysis) PA1 T=weight terephthalic acid/weight water PA4 (after hydrolysis) PA1 Pressure: 400, 900 psi PA1 M: 2, 20 m/m PA1 T: 5, 30 wt/wt PA1 a=0.867 PA1 b=140.9 PA1 c=-8.44 PA1 d=-3.4
As used herein, the term "oligomers" is used to include not only molecules having repeating units, but also half acid adducts, which in the case of polyethylene terephthalate is MHET, and full acid adducts, which in the case of polyethylene terephthalate is p-bis(2-hydroxyethyl) terephthalate (BHET). Such half acid and full acid adducts are the greatly predominating oligomers found in the hydrolysis of condensation polymers.